In lithographic printing, ink receptive regions, known as image areas, are generated on a hydrophilic surface. When the surface is moistened with water and ink is applied, the hydrophilic regions retain the water and repel the ink, while the ink receptive regions accept the ink and repel the water. The ink is then transferred to the surface of suitable materials upon which the image is to be reproduced. In some instances, the ink can be first transferred to an intermediate blanket that in turn is used to transfer the ink to the surface of the materials upon which the image is to be reproduced.
Lithographic printing plate precursors useful to prepare lithographic (or offset) printing plates typically comprise one or more imagable layers applied over a hydrophilic surface of a substrate (or intermediate layers). The imagable layer(s) can comprise one or more radiation-sensitive components dispersed within a suitable binder. Following imaging, either the exposed regions or the non-exposed regions of the imagable layer(s) are removed by a suitable developer, revealing the underlying hydrophilic surface of the substrate. If the exposed regions are removed, the element is considered as positive-working. Conversely, if the non-exposed regions are removed, the element is considered as negative-working. In each instance, the regions of the imagable layer(s) that remain are ink-receptive, and the regions of the hydrophilic surface revealed by the developing process accept water or aqueous solutions (typically a fountain solution), and repel ink.
“Laser direct imaging” methods (LDI) have been known that directly form an offset printing plate or printing circuit board using digital data from a computer, and provide numerous advantages over the previous processes using masking photographic films. There have been considerable developments in this field from the use of more efficient lasers, and improved imagable compositions and components thereof.
Various radiation-sensitive compositions are known for use in negative-working lithographic printing plate precursors as described for example in U.S. Pat. No. 6,309,792 (Hauck et al.), U.S. Pat. No. 6,893,797 (Munnelly et al.), U.S. Pat. No. 6,727,281 (Tao et al.), U.S. Pat. No. 6,899,994 (Huang et al.), and U.S. Pat. No. 7,429,445 (Munnelly et al.), U.S. Patent Application Publications 2002/0168494 (Nagata et al.), 2003/0118939 (West et al.), and EP Publications 1,079,276A2 (Lifka et al.) and 1,449,650A2 (Goto et al.). In addition, U.S. Pat. No. 7,429,445 (Munnelly et al.) describes on-press developable negative-working lithographic printing plate precursors that contain various infrared radiation absorbers.
Negative-working lithographic printing plate precursors often contain high amounts of free radical-producing initiator compounds to increase imaging speed (improve imaging sensitivity). Some useful initiator compounds used in this manner are iodonium compounds having tetraaryl borate counter ions (such as tetraphenyl borate counter ions) as described for example, in U.S. Pat. No. 6,645,697 (Urano) and WO 2008/150441 (Yu et al.).
However, many of the cationic cyanine dyes that are used as infrared radiation absorbers in such lithographic printing plate precursors exhibit a strong tendency to crystallize in the presence of borate anions that can be in the imaging formulations. This crystallization leads to reduced shelf life of the imagable layer composition in the lithographic printing plate precursor before the precursor is imaged. The crystallization also leads to problems during manufacturing of the imagable layer formulations.
There is a need to improve imaging speed using borate-ion containing initiator compositions in negative-working lithographic printing plate precursors without the undesirable crystallization that causes the noted problems.